My 93 240 4 cylinder 440,000 km has a oil leak on the left hand side of the engine under the intake manifold. I cleaned the engine and it looks like its leaking from the head gasket area behind the black breather box its hard to see.
Oil is running down the side of the block. Is it common for these cars to leak oil from the head over time.

I've not seen a bad head gasket that elicited "Oil is running down the side of the block."

I'd suspect 1) breather box; 2) rear cam plug that's somehow wicking oil onto the side of the block.

I'd pull the intake manifold and inspect closely. Breather boxes can crack at the seam and leak oil.
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240 drivers / parts cars - JH, Ohio

What leads you to believe it's not at the breather box? Check your flame trap, you could be blowing oil out due to crankcase pressure.

I have the little flame trap taken out, its so hard to see where the oil is coming from is all over the starter side of the engine, the rear cam plug is very oily in that area.

"the rear cam plug is very oily in that area."
It could be cam plug is leaking or about to fall out.
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1980 245 Canadian B21A with SU carb, M46 trans, 3:31 dif, in Brampton, Ont.

Finally found a oil leak at the bottom of the rear cam plug seal. Took off the rocker cover and then the rear cam seal retainer pulled the seal out its just like a hard plastic disc cleaned everything and put it back in with some sealer around it, tightened it all up no more oil leak, I have ordered a new oil trap box and seal when in i will install it. Do you have to take off the intake manifold to replace the oil trap box, i thought i could take out the iac valve and intake duct and get it out that way remember my car is Right Hand drive.

When I replaced my oil trap and hoses a few months ago, I removed the throttle body and everything forward of it to the air box, including the IAC. This gave me a good opportunity to clean everything up. The intake manifold stayed on the block - no issues with access to the oil trap.

Got the oil separator out today, looks like i found the oil leak o ring seal on the bottom was flat and hard, top outlet on the box was blocked. Have ordered a new box and o ring. Cleaned up the old box with degreaser and spayed it with the gernie through the outlets the amount of crap that came out was unbelievable.

Your car is close to a potentially disastrous situation.

If it were my car, I'd clean or replace the breather box and replace ALL hoses associated with the breather box/flame trap. Those hoses can clog. The small hose from the flame trap junction to the intake manifold clogged on my 90 240 and it took me a long time to find it. I had a small RMS seal leak that was cured by replacing that hose. You want the breather system to be flowing easily. No restrictions.

I'd also replace the rear cam plug. If the plug blows out, the car is basically not driveable. FYI - IPD (ipdusa.com) makes a small retainer to hold that plug in.

Could it be the breather box itself is leaking? Is the oil leak below it?

Yes, with the number of km's on your engine it is entirely possible the head gasket is leaking. However, they normally leak oil into the antifreeze. Have you checked your antifreeze overflow tank for signs of oil contamination?